This fort may have been built in the 16th century by the Portuguese although doing a search for its history seems to show conflicting information. It was certainly used in the 17th century by Saif bin Sultan Al Ya’rubi. The distinctively shaped tower was constructed to deflect cannonballs; the fort was a lot closer to the sea than it is today and it had a ditch on at least three sides, that would fill with sea water.
Some interesting history from the book: The Ya’rubi dynasty of Oman. Raymond Denis Bathurst.
Sultan bin Saif Al Ya’rubi defeated Portuguese troops, who had been occupying certain coastal dominions. This liberated Omani trade and ushered in nearly a century of relative wealth and progress, until the death of Sultan ibn Sayf II and resultant break out of civil war in 1718.
The Al-Ya’ariba trace their descent from Ya’arab bin Kahtan, whom some date to about 800 BC. The family originated in Yemen and belonged to the Ghafiri faction. Nasir bin Murshid bin Sultan al Ya’Aruba (r. 1624-1649) was the first Imam of the Yaruba dynasty, elected in 1624. He moved the capital to Nizwa, the former capital of the Ibadhi Imamate. Nasir bin Murshid was able to unify the tribes with a common goal of expelling the Portuguese. He built up the Omani army and took the main towns as well as the forts of Rustaq and Nakhal. His forces threw the Portuguese out of Julfar (now Ras al-Khaimah) in 1633. In 1643 they took the fort at Sohar.
Nasir bin Murshid was succeeded by Sultan bin Saif (r. 1649-1688), his cousin. Sultan bin Saif completed the task of expelling the Portuguese. He captured Sur, Qurayyat and Muscat, expanded the fleet and attacked the Portuguese on the Gujarat coast. Under Sultan bin Seif and his successors Oman developed into a strong maritime power.The Omanis took many of the Portuguese possessions in East Africa. The first attack on Zanzibar, then held by the Portuguese, was in 1652. In 1660 Omani forces attacked Mombasa, forcing the Portuguese to take refuge in Fort Jesus. There was continued fighting between the forces of Portugal and Oman in the East African coast in the years that followed.
Bil’arab bin Sultan (r. 1679-1692) succeeded as Imam in 1679 after the death of his father, Sultan bin Saif. This confirmed that the succession was now hereditary, since his father had also succeeded dynastically, while in the Ibadi tradition the Imam was elected. Most of his reign was occupied in a struggle with his brother, Saif bin Sultan, who succeeded Bil’arab bin Sultan when he died at Jabrin in 1692.